The Cladoniaceae are a family of lichen-forming fungi in the order Lecanorales, comprising about 560 species distributed amongst 18 genera. This family is one of the largest among lichen-forming fungi and is globally distributed, from Arctic tundra to tropical rainforests, favouring humid environments while being intolerant of arid conditions. Molecular phylogenetics has substantially advanced the understanding of their complex taxonomic history, revealing intricate evolutionary relationships and leading to a refined classification. Notable members include reindeer moss and cup lichens of the genus Cladonia, which consist of about 500 species and forms a major part of the diet for large mammals in taiga and tundra ecosystems.

A distinctive feature of many Cladoniaceae species is their dimorphic thallus: a scaly or crust-like primary form and a fruticose (shrub-like) secondary form known as a podetium or pseudopodetium. These lichens typically grow on soil, decaying wood, or tree trunks, with a few species found on rocks. They form symbiotic associations with green algae, usually from the genus Asterochloris. The family is known for its diverse secondary metabolites—over 70 have been identified—which play roles in species identification and ecological functions such as protection against UV radiation and herbivore deterrence. Genomic studies have uncovered considerable variation in mitochondrial DNA among Cladoniaceae species, and this variation has helped clarify relationships within the family.

Some Cladoniaceae species have economic value, particularly in decorative uses such as floral arrangements and model-making due to their unique structures. Six species are included in the International Union for Conservation of Nature's Red List of Threatened Species, facing threats from habitat loss, climate change, and human activities. Conservation efforts are ongoing to protect these vulnerable species.

Before the term 'lichen' was widely adopted, one of the earliest classification systems for these organisms was developed by the German botanist Johann Jacob Dillenius. In his 1741 system, Cladonia species were placed within the genus Coralloides. In his Species Plantarum, Carl Linnaeus formally described several species that are now classified within the Cladoniaceae. Although he initially placed them in the eponymously named genus Lichen, a dozen of these species are recognised as belonging to the genus Cladonia. Among these was Cladonia pyxidata, a representative of the "pixie cup" or "trumpet" lichens—so named for their distinctive shape, as well as some well-known and widespread Cladonia species: C. coccifera, C. cornuta, C. deformis, C. digitata, C. fimbriata, C. gracilis, C. portentosa, C. rangiferina, C. stellaris, C. subulata, and C. uncialis. These 12 species represent about 14% of the 80 Lichen species Linnaeus described in his seminal 1753 work.

The German naturalist Jonathan Carl Zenker formally introduced the Cladoniaceae to science in 1827, with his work appearing in a publication edited by Karl Goebel. Zenker's initial concept of the family included genera such as Baeomyces, Icmadophila, and Stereocaulon, which have since been reclassified into separate families due to advances in taxonomic understanding. William Nylander included 53 Cladonia species worldwide in his 1860 work Synopsis lichenum. When Edvard August Vainio published his three-volume monograph on the Cladoniaceae (Monographia Cladoniarum universalis, 1887, 1894, and 1897), he included 134 species and subspecies. In his circumscription of the family, the genera Pycnothelia, Cladia, and Cladina were included in the genus Cladonia. In recent history, Teuvo Ahti's extensive research, including comprehensive monographs and revisions, has considerably advanced the understanding of Cladoniaceae taxonomy and biogeography.

Cladoniaceae is now one of the largest families of lichen-forming fungi, with about 560 species distributed amongst 18 genera. The type genus is Cladonia, circumscribed by the Irish physician and botanist Patrick Browne in 1756. He included eight species in his new genus. Of their occurrence, he wrote: "All these species are found in great abundance in the mountains of Liguanea: they grow mostly on the ground, among other sorts of moss, but a few ... species chiefly are found upon the decaying trunks of trees."

As is standard practice in botanical nomenclature, the name Cladoniaceae is based on the name of the type genus, Cladonia, with the ending -aceae indicating the rank of family. The genus name comprises the Greek word: κλάδος (klādos), meaning "branch", "bud", or "shoot"; and the Latin -ia, a suffix commonly used in Latin to form nouns, particularly in taxonomy to denote genera.

It is well known that reindeer feed on lichens, which has led to the widespread but misleading name "reindeer moss". This common name, along with "reindeer lichen" and "caribou lichen", is typically applied to the ground-dwelling, mat-forming species that were previously classified in the genus Cladina. Cladonia species with cup-shaped structures borne at the tips of vertical stalks (podetia) are often known as "pixie cups". Examples include the "boreal pixie cup" (C. borealis), the "finger pixie cup" (C. digitata), and the "red-fruited pixie cup" (C. pleurota) Additional names alluding to these characteristic structures include the "powdered funnel lichen" (C. cenotea), and the "trumpet lichen" (C. fimbriata). Some names reference the reddish hue of their fruiting structures, such as "British soldiers" (C. cristatella), the "jester lichen" (C. leporina), and the "lipstick powderhorn" (C. macilenta).